Hastening the Onset of the Breeding Season

Julie Skaife
Select Breeders Services

The horse is a seasonal breeder meaning that natural mating occurs during certain times of the year to ensure that the timing of birth is optimal for survival with regard to ambient temperature, food and water availability, and even changes in the predation behaviors of other species. Alternative breeding behaviors used by animals include opportunistic breeders that mate whenever the conditions of their environment become favorable and continuous breeders that mate year-round. Seasonal breeders are controlled by the length of daylight, i.e. the photoperiod, and can be divided into long day breeders, e.g. the horse, that start cycling when the days get longer (spring) and short day breeders, e.g. deer, that start cycling when the days get shorter (fall). The length of photoperiod can be manipulated in order to hasten the onset of the breeding season. This is a popular management technique used in Thoroughbred breeding, so foals can be born as close to the standardized January 1st birth date as possible. It is also common with show or halter horses, with the goal being to maximize maturity for age determined competition.


Putting Your Mare Under Lights
Breeding season in the Northern hemisphere mare typically runs from late spring (April) through early fall (September). The mare, being seasonally polyestrous, has multiple heat cycles during the reproductive season. During the winter months (mid-Nov to mid-Feb) the mare is considered to be anestrous, demonstrating no behavioral signs of sexual receptivity and a failure to develop follicles that ovulate. Although it is estimated that 75-85% of mares are truly anestrous, it is possible that the remaining mares may show some estrous activity, albeit erratic and usually anovulatory. Either side of anestrous the mare is in a transitional period that is also characterized by inconsistent heat cycles with inconsistent ovulations. Year round estrous behavior can be seen in mares closer to the equator.

Females are typically more “seasonal” than their male counterparts. Thus, unlike mares that enter an anestrous period, stallions remain fertile year-round, usually showing only a mild decline in sexual behavior and sperm production out of season. Consequently manipulation of the photoperiod is predominantly targeted to the mare and is accomplished by “putting the mare under lights”, which is (almost) as simple as it sounds. Supplementing the length of “daylight” with artificial lighting is referred to as phototropic stimulation. The mare should begin to cycle 50 to 60 days after the beginning of a lighting program. So, beginning a lighting program on December, 1st will allow for normal estrus cycles and breeding during February. The results of this lighting program usually allow for a foal to be born in January of the following year.


Lighting Programs
There are a number of different lighting programs that can be used to accomplish the same goal. The first and most preferred method is to provide from 14-1/2 to 16 hours of continuous light each day. If the mare is turned out during the day she must be brought in and put under artificial lights before the natural light outside fades. If you want to coordinate the breeding cycles of a group of mares, this can be accomplished by lighting their shared paddock.
Lighting can be started abruptly on December 1st or can be increased gradually over a period of 60 days. For example, under the constant regime the lights are turned on from 4.30pm to 10 or 11pm for 10 weeks prior to the desired breeding date. With the stepwise regime 3 hours of light are added the first week and then another 1/2 hour of light each additional week until a 15-16 hour “day” is achieved.

Another lighting plan involves turning on the indoor lights, for a short period, starting nine hours after sunset, and continuing that program until the nine hours coincides with sunrise. However, since the time of the sunset is constantly changing, this program requires routine management to make sure you change the timing of the lighting to coincide with the changing sunset. Success has also been achieved with turning on the lights for as little as two hours when programmed to occur 9 hours after sunset. Whatever lighting program you decide to utilize, consider using an automatic timer to save both time and money.


How much light is required?
A critical factor is the amount of light that must be provided to simulate daylight. A minimum of 10 footcandles of light (107 lux) is required, either incandescent or fluorescent. A good rule of thumb is to provide enough light to comfortably read the small print in a newspaper seated anywhere in the stall. A 100 watt light bulb (or two 40 watt bulbs) provides enough light for the average sized stall.


Additional Considerations
The lighting program is usually discontinued once the mare is cycling and ovulating regularly. Research suggests that a period of short days is required prior to entering a photostimulation period, so there is no advantage to starting your lighting program earlier than November and a year round lighting plan may actually be disadvantageous. In this respect, if the lights go out and the mare is subjected once again to short days (even for as few as 3 days), this may reset the system, so care must be taken to ensure the lighting regime is maintained. Research also suggests that ambient temperature may play a secondary role in seasonality, so mares in colder climates that are exposed to lengthening daylight may start cycling later than mares similarly exposed in warmer climates. Remember, that not only does the lighting regime hasten the onset of the breeding season; it will also cause early shedding of the winter coat, so don’t forget to bear this in mind for those mares that spend all or part of their time outside and provide blanketing in colder climates if required.


Foaling Mares
Lighting programs also have an effect on your foaling mares. Research has shown that foaling mares, exposed to 16 hours of daylight during the last portion of their gestation, will foal on average 7-10 days prior to mares who are not exposed to lights. More importantly, if you plan on breeding the mare again following her foaling, a lighting program will helps prevent her from entering a post-foaling anestrous period that would coincide with a natural anestrous period. If foaling mares are not exposed to a lighting program, a proportion may not cycle until March or April when the natural breeding season begins.


How it Works
The hypothalamus of the brain is considered to be the central endocrine or hormonal control for reproductive activity. Hence, factors that determine when a seasonal breeder will be ready for mating affect this portion of the brain. In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei and superior cervical ganglion to the pineal gland. The pineal gland produces melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonality. The production of melatonin, also know as the “hormone of darkness”, by the pineal gland is stimulated by darkness and inhibited by light. Melatonin has an inhibitory effect on the production of gonadotropin releasing hormone (GnRH) by the hypothalamus in long day breeders. The decrease in melatonin production due to longer days in the spring, results in a reduction in the suppression of the hypothalamus and thereby an increase in GnRH pulses. GnRH subsequently promotes an increase in the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are called gonadotropins because they stimulate the gonads - in males, the testes, and in females, the ovaries. In mares, these hormones, LH and FSH, control the cyclic nature of estrous periods by promoting follicular development and ovulation.


Hormonal Therapy
Lighting programs for mares are the most common method for advancing the breeding season. There is, however, extensive research for alternative methods that would not require 45-60 days of artificial light. Hormonal therapies using various forms of FSH, GnRH analogs, prolactin, or dopamine antagonists have been proposed and shown various degrees of success in stimulating a fertile estrus period during winter anestrus. Speak with your veterinarian or call your local SBS-affiliate for more information regarding these historical therapies and some potential new therapies that are promising.
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A lighting program is a cheap and effective way to manage your mare’s seasonality, but remember that it doesn’t shorten the transitional period of the mare’s reproductive cycle, it just advances it forward in the calendar, so be sure to start the lighting regime 50-60 days before the date you would like to get your mare bred. Also, a good practice in reproductive management is to perform a pre-breeding exam on the first heat cycle of the year. If your aim is for a January foaling date, then lighting should commence in December. The efficiency of any lighting regime is subject to a number of factors, e.g. lack of sufficient lighting, interruption of lighting, significantly low ambient temperatures etc., and therefore must be well managed to be successful and may benefit from combination with hormonal therapy as well.